For a small Amazonian frog called Rhinella proboscidea, death is no impediment to sex. The males form huge mating balls in which dozens of individuals compete to fertilise a female. These competitions are so intense, and the combined males so heavy, that the poor female sometimes drowns in the struggle.

But for the males, that’s not a deal-breaker. Thiago Izzo from Brazil’s National Institute of Amazonian Research has found that the males can force the eggs from the bodies of the deceased female, and fertilise them. It’s a unique strategy and one that effectively involves sexual reproduction with a dead partner. Izzo calls “functional necrophilia”.

R.proboscidea is a small frog that looks like a dead leaf, right down to its pointed snout, its brown colour and the central white ‘vein’ running down its back. But its camouflage breaks down when it’s time to mate. Hundreds of males gather at breeding sites for just two to three days and when any female shows up, there’s intense competition for her attention. This strategy is called “explosive breeding” and it’s as violent as it sounds. Males wrestle for mating rights, and will try to displace any rivals that have actually found a female. The result is a large mating ball with a female at its bottom. She often drowns.

In the deadpan style of academics, Izzo writes that “such occurrences are obviously detrimental to females”. You don’t say.

He has seen the aftermath of the carnage first-hand, having found several explosive breeding sites in Brazil’s Adolfo Ducke Forest Reserve between 2001 and 2005. The first time, he found around 100 males and 20 dead females. The second time: 50 males and 5 dead females. But when Izzo dissected the females, he couldn’t find any eggs inside them. Where had they gone?

Izzo found the answer when he saw a male grasping the body of a dead female and rhythmically squeezing the sides of her belly. Out popped her eggs, like beads on a jelly-coated string.

The male on top squeezes eggs out of the dead female below. From Izzo et al, 2013. Journal of Natural History

Izzo saw the same behaviour again and again. On one occasion, the male pushed his dead partner around the pond, “apparently to avoid other males”. The eggs that emerge are quickly fertilised—Izzo kept an eye on them and saw that they eventually developed into embryos.

There have been many other cases of animals having sex with the dead, including several frogs and an ameiva lizard that tried to mate with a road-killed female. On Scott’s Antarctic expedition of 1910-1913, George Murray Levick saw a male Adelie penguin trying to have sex with a dead female, an act of “astonishing depravity” that he removed from his paper on the penguin’s behaviour.

All of these incidents were regarded as mistakes, but the behaviour of R.proboscidea is anything but. Izzo expects that other explosive breeders—and there are many frogs that do this—might also rely on functional necrophilia.

This behaviour clearly benefits the male. He doesn’t waste the huge amount of energy that he just spent on wrestling with other suitors. It would be a wasted effort, in any case—R.proboscidea males outnumber the females by ten to one, so the odds of finding and mating with another partner are pretty low. But he doesn’t have to. He can still foster a new generation of frogs.

There might even be some benefits for the female. She too gets a post-mortem chance of passing her genes to future frogs despite the unfortunate side effect of, er, being drowned by a ball of violent males. Silver lining!

First exploding duck genitals, then poop-exploding face mites, now Kermit the necrophiliac frog. Depending on a reader’s viewpoint, you are either ruining nature or making it totally flipping awesome (this reader is in the latter category, of course).

The paragraph “There might even be some benefits for the female. She too gets a post-mortem chance of passing her genes to future frogs despite the unfortunate side effect of, er, being drowned by a ball of violent males. Silver lining!”
… is totaly mysogynistic!! There is nothing beneficial to a female about being killed in a orgy. What is wrong with you???

I’m curious about that sex ratio as well. Why is it so lopsided? I’m also rather worried about other violent effects of lopsided sex ratios – for humans, in places like India where life is already tough and violent for women!

The sex ratio thing could easily be solved by breeding them in captivity and only releasing females. We (humans) could flood the population with females so they aren’t so gruesomely massacred and the males aren’t so desperately psychotic.

2) The sex ratio thing – it’s really not that unusual! All sorts of factors contribute to massively skewed sex ratios in the wild, including parasites, predators, differing investment by parents, differing competition between the sexes.

The bizarre thing is mapping human values onto this (“psychotic”??), seeing skewed sex ratios as a problem to be fixed, and actually suggesting that we deliberately skew the sex ratios back to what we think is appropriate.

Ed:
Izzo is probably looking at the operational sex ratio: the number of males ready to mate when gravid females show up at mating sites. Females spawn all their eggs rather quickly and, if they survive, return to the forest; males stick around and mob each female as she arrives. In two or three nights, as many females may arrive individually as there are males waiting. Also, there is no guarantee that all females will be ready to mate in a given mating session. This would further skew the operational sex ratio even if males and females were equally numerous.
That said, it seems reasonable to assume that female deaths further skew the operational sex ratio towards males.

I’m not sure why these findings are surprising at all. Many male frogs squeeze females during amplexus to either encourage the female to initiate ovipositing or to physically push the eggs out of her. The males probably have no idea that the females are dead – they are just following their normal instinct to grab a female in amplexus and squeeze. To identify this as if it is some sort of newly discovered behavior is a stretch. And naming it functional necrophilia? This is external fertilization we’re talking about! In frogs! It’s not as if they’re picking corpses on purpose.

As far as this behavior contributing to a skewed sex ratio in the general population, did you read the article? The article states that males are found dead at a rate of 10:1 to 20:1 over that of females after these mating events. If anything, this should skew the population towards females. Something else is going on here.

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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